Measurement Modes

Due to the nature of Geotechnical measurements the acquisition of sensor data is relatively slow compared to other aspects of data acquisition operations. The VibWire-108 does support only  Measurement-upon-demand mode of operation for SDI-12 / 485 networks and a continuous mode only when the analogue output port is being used.    


Measurement on demand


The VibWire-108 is idle (held in a low power mode) until it is woken by the data logger sending it a measurement request. The  interface then takes the measurement returns the data. The time taken to return data to the DT80 logger is proportional to the operating frequency of the vibrating wire sensors. The lower the operating frequency then the longer the delay before a new value is available for reading. The maximum delay is 60 seconds from issuing the measurement request to reading new values.

Measurement on demand minimises the overall system power usage and also reduces wear on the coil used with the vibrating wire sensor. In order to operate in Measure on demand mode the DT80 must first send a measurement request instruction and wait VibWire-108 to complete the measurement operation before moving on to the next instrument. It is not currently possible for other SDI-12 tasks to be carried out while the DT80 is waiting for the data to be returned from the VibWire-108.

Either of the  aM! or aC! commands will start the VibWire to make a scan of the input channels.


Continuous Measurement - Analogue Output Only


The analogue output port is used only when the VibWire-108 is connected to an analogue data acquisition system. The vibrating wire sensor inputs have to be manually defined and the results scaled to engineering units within the logger unit. The results can be observed faster than those reported over an SDI-12/485 network but there are errors inherent in the digital-analogue and analogue-digital conversion processes involved.  

The output ports within VibWire-108 can be connected to the DT80 analogue data acquisition system when relatively fast results are required.


Firmware Identification


SDI-12 applications running within the DT80 can take advantage of the features available for intelligent sensors just like the VibWire-108 and can interrogate the instruments for their identifier details. In the case of the VibWire-108 the ID is by default

KEYNESCOVibWire-1080001         -- represents instrument with firmware release 1
where the last three digits represent the firmware version number.  

Registers

The VibWire-108 supports 8 vibrating wire sensors and 8 analogue input channels and all these values can be accessed by the DT80. The VibWire-108 divided it’s set of registers in to a number of register sets and each set holds 4 values. The data format is:

aD0!  = channel 0 and 3       ‘aD0!’  -- Vibrating Wire inputs 0 - 3
aD1! =  channel 4 and 7      ‘aD1!’  -- Vibrating Wire inputs 4 - 7
aD2! =  channel 0 and 3   Therm/analog  ‘aD2!’  -- Temp/current loop inputs 0 - 3   (values in mV)
aD3! =  channel 4 and 7   Therm/analog  ‘aD3!’  -- Temp/current loop inputs 4 - 7   (values in mV

Instrument responds: ‘a+xxxx.x+xxxx.x+xxxx.x+xxxx.x\r\n’      xxxx.x   is the format of the number returned - 1 decimal place
        

VibWire-108 Sensor Scanning & Start-up Operations


The VibWire-108 is a stand-alone interface using embedded software as such boots up as soon as power is applied. In practice the instrument starts to operate as soon as the initial “BASIC” menu can be observed on the seven segment display. The “BASIC” menu is displayed prior to the instrument starting any operations.

The VibWire-108 remembers any preset configuration and will restart in that condition upon being powered on. This enables groups of units to have their ID numbers and network ports preset so that under control from the logger units they can be powered on in turn, used to acquire data and switched off with out need to reset any comms etc..

The start up time from powering on the unit to being able to send instructions in approximately 1 sec.

The time taken from issuing an instruction to the VibWire-108 to the unit sending a reply from power on is approximately 2 secs.

The VibWire-108 uses a phase locked loop and resonant frequency tracking system to operate the ping for vibrating wire sensors. This circuit settles almost immediately after the first ping to a sensor. The ability to use a phase locked ping  ensures the minimum amount of energy is used to drive the sensors as they are always being driven very close to resonance.